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[clang] Add -Wunused-but-set-global (#188291)
Commit fd11cf4 extended `-Wunused-but-set-variable` to static globals. To make it easier for downstream projects to integrate this new functionality, this commit introduces `-Wunused-but-set-global` so it can be easily disabled as projects investigate and fix new findings.
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‎clang/docs/ReleaseNotes.rst‎

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@@ -231,7 +231,10 @@ Attribute Changes in Clang
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Improvements to Clang's diagnostics
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-----------------------------------
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- ``-Wunused-but-set-variable`` now diagnoses file-scope variables with
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internal linkage (``static`` storage class) that are assigned but never used. (#GH148361)
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internal linkage (``static`` storage class) that are assigned but never used.
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This new coverage is added under the subgroup ``-Wunused-but-set-global``,
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allowing it to be disabled independently with ``-Wno-unused-but-set-global``.
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(#GH148361)
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- Added ``-Wlifetime-safety`` to enable lifetime safety analysis,
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a CFG-based intra-procedural analysis that detects use-after-free and related

‎clang/include/clang/Basic/DiagnosticGroups.td‎

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@@ -1158,7 +1158,9 @@ def UnusedValue : DiagGroup<"unused-value", [UnusedComparison, UnusedResult,
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def UnusedConstVariable : DiagGroup<"unused-const-variable">;
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def UnusedVariable : DiagGroup<"unused-variable",
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[UnusedConstVariable]>;
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def UnusedButSetVariable : DiagGroup<"unused-but-set-variable">;
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def UnusedButSetGlobal : DiagGroup<"unused-but-set-global">;
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def UnusedButSetVariable
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: DiagGroup<"unused-but-set-variable", [UnusedButSetGlobal]>;
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def UnusedLocalTypedef : DiagGroup<"unused-local-typedef">;
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def UnusedPropertyIvar : DiagGroup<"unused-property-ivar">;
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def UnusedGetterReturnValue : DiagGroup<"unused-getter-return-value">;

‎clang/include/clang/Basic/DiagnosticSemaKinds.td‎

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@@ -433,6 +433,9 @@ def warn_unused_variable : Warning<"unused variable %0">,
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InGroup<UnusedVariable>, DefaultIgnore;
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def warn_unused_but_set_variable : Warning<"variable %0 set but not used">,
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InGroup<UnusedButSetVariable>, DefaultIgnore;
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def warn_unused_but_set_global : Warning<"variable %0 set but not used">,
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InGroup<UnusedButSetGlobal>,
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DefaultIgnore;
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def warn_unused_local_typedef : Warning<
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"unused %select{typedef|type alias}0 %1">,
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InGroup<UnusedLocalTypedef>, DefaultIgnore;

‎clang/lib/Sema/Sema.cpp‎

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@@ -1610,7 +1610,7 @@ void Sema::ActOnEndOfTranslationUnit() {
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emitAndClearUnusedLocalTypedefWarnings();
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}
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if (!Diags.isIgnored(diag::warn_unused_but_set_variable, SourceLocation())) {
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if (!Diags.isIgnored(diag::warn_unused_but_set_global, SourceLocation())) {
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// Diagnose unused-but-set static globals in a deterministic order.
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// Not tracking shadowing info for static globals; there's nothing to
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// shadow.

‎clang/lib/Sema/SemaDecl.cpp‎

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@@ -2245,8 +2245,13 @@ void Sema::DiagnoseUnusedButSetDecl(const VarDecl *VD,
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return;
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}
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unsigned DiagID = isa<ParmVarDecl>(VD) ? diag::warn_unused_but_set_parameter
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: diag::warn_unused_but_set_variable;
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unsigned DiagID;
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if (isa<ParmVarDecl>(VD))
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DiagID = diag::warn_unused_but_set_parameter;
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else if (VD->isFileVarDecl())
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DiagID = diag::warn_unused_but_set_global;
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else
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DiagID = diag::warn_unused_but_set_variable;
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DiagReceiver(VD->getLocation(), PDiag(DiagID) << VD);
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}
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‎clang/test/C/C2y/n3622.c‎

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@@ -1,7 +1,7 @@
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// RUN: %clang_cc1 -verify=good -pedantic -Wall -Wno-unused-but-set-variable -std=c2y %s
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// RUN: %clang_cc1 -verify=compat,expected -pedantic -Wall -Wno-unused-but-set-variable -Wpre-c2y-compat -std=c2y %s
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// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-variable -std=c23 %s
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// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-variable -std=c17 %s
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// RUN: %clang_cc1 -verify=good -pedantic -Wall -Wno-unused-but-set-global -std=c2y %s
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// RUN: %clang_cc1 -verify=compat,expected -pedantic -Wall -Wno-unused-but-set-global -Wpre-c2y-compat -std=c2y %s
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// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-global -std=c23 %s
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// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-global -std=c17 %s
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// good-no-diagnostics
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/* WG14 N3622: Clang 22

‎clang/test/Misc/warning-wall.c‎

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@@ -84,6 +84,7 @@ CHECK-NEXT: -Wpotentially-evaluated-expression
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CHECK-NEXT: -Wunused-variable
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CHECK-NEXT: -Wunused-const-variable
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CHECK-NEXT: -Wunused-but-set-variable
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CHECK-NEXT: -Wunused-but-set-global
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CHECK-NEXT: -Wunused-property-ivar
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CHECK-NEXT: -Wvolatile-register-var
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CHECK-NEXT: -Wobjc-missing-super-calls

‎clang/test/Sema/warn-unused-but-set-static-global.c‎

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@@ -1,4 +1,6 @@
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// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -I %S/Inputs -verify %s
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// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -Wno-unused-but-set-global -I %S/Inputs -verify=no-global %s
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// no-global-no-diagnostics
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// Test that header-defined static globals don't warn.
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#include "warn-unused-but-set-static-global-header.h"

‎clang/test/Sema/warn-unused-but-set-static-global.cpp‎

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@@ -1,4 +1,6 @@
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// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -verify -std=c++17 %s
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// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -Wno-unused-but-set-global -verify=no-global -std=c++17 %s
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// no-global-no-diagnostics
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static thread_local int tl_set_unused; // expected-warning {{variable 'tl_set_unused' set but not used}}
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static thread_local int tl_set_and_used;

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